In a co-generation system, a reciprocating engine type electric generator has been conventionally utilized, which generates electricity by use of a reciprocating engine. This type of reciprocating engine type electric generator is disclosed in Japanese Patent laid-Open Publication No. 2002-238167. This reciprocating engine type electric generator includes a reciprocating engine having a piston, an electric generator having a stator coil and a rotor and generating an alternate voltage at a stator coil following rotation of the rotor, and an engine starter. The rotor of the electric generator is connected to the piston of the reciprocating engine via a crankshaft. Therefore, the rotor is operated in association with a movement of the piston.
The reciprocating engine is applied with a relatively large load when it is initially activated. Therefore, the starter is required for the purpose of starting the engine. More. particularly, the starter starts the reciprocating engine by rotating a ring gear at a minimum rotational speed. When the engine rotational speed is stabilized after a while, the starter operation is stopped, and electricity is extracted from the electric generator. In general, the starter is employed only when the reciprocating engine is initially activated, and the starter operation is stopped when the engine rotation is stabilized.
According to the above-described technology, when the reciprocating engine is started, the starter is necessary to stabilize the engine rotation. However, the starter and components attached thereto may cause increase of the manufacturing cost of the reciprocating engine. Further, in light of characteristics of the starter, it is necessary to maintain the starter and the components attached thereto.
Meanwhile, when a brushless DC motor is activated, electric voltage to be applied to the stator coil is controlled in response to a rotational position of a rotor of the motor. Therefore, it is indispensable to obtain the rotational position of the rotor. In this case, a sensor such as a Hall element is equipped adjacent to the rotor and detects the rotational position of the rotor. However, when the sensor is equipped as described above, wire interconnection around there may become complicated. Further, a working temperature of the sensor, and further the environment in which the motor is used may be restricted.
In light of foregoing, recent developments have led to a technology by which the brushless DC motor is controlled without employing the sensor such as the Hall element, i.e., in a sensor-less control manner. However, when the brushless DC motor is employed for activating the reciprocating engine, it seems to be difficult for the brushless DC motor to be controlled properly without employing the sensor. The background of this result is described below.
When the piston of the reciprocating engine heads to a top dead center, the rotational speed of the rotor connected to the piston is decreased rapidly and instantly. While the piston has exceeded the top dead center, the rotational speed of the rotor is increased rapidly and instantly. Therefore, the information of the rotor rotational position relatively widely fluctuates when the piston is operated in the vicinity of the top dead center.
A need thus exists for providing a reciprocating engine type electric generator that does not require a starter and components attached thereto, a sensor such as a Hall element, and can prevent a rotor rotational speed from deviating from a rotational magnetic field generated at a stator coil.